dark matter

In astronomy and cosmology, dark matter is a type of matter hypothesized to account for a large part of the total mass in the universe. Dark matter cannot be seen directly with telescopes; evidently it neither emits nor absorbs light or other electromagnetic radiation at any significant level.[1] Instead, its existence and properties are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe. Dark matter is estimated to constitute 84% of the matter in the universe and 23% of the total energy density (with almost all the rest being dark energy).[2]
http://en.wikipedia.org/wiki/Dark_matter